Radio amplifying circuit



Nov. 16, 1943. R. DE coLA 2,334,670

' RADIO AMPLIFYING cIRcUIT` Filed Dec. 15, 1940 2 sheets-sheet 1 GAIN GA I N Nov. 16, 1943.

R, DE COLA RADIO AMPLIFYING CIRCUIT Filed D90. 15, 1940 2 sheetsLsheet 2 55o FREQ. K C. |650 j I iff' FREQ.K.C.

yez? n Patented Nov. 16, 1943 l Rinaldo De Cola, Chicago, lI Il., Vassigifor folliclniont Radio Copotton, Chicago, IIII., a.' corporrtioli of Illinois Application neeemhe'r 13, 1940, seriarNowroa 6 claims. (c1. raul-40) This invention relates to radio frequency cir# cuits and more particularly -to radio frequency ampliers of the permeability'tuned type.

In permeability tuned circuits ofthe prior art, considerable difficulty hasv been 'encountered in obtaining a uniform radio frequency response and high overall selectivity over a complete tun# ing range. In' some instances of the prior art, a uniform response as a function of the frequency has" been provided by ymaintaining uniform ,the L/R ratio of the Ypermeability tuned coil, `L/R.l being maintained' constant by a suitable construcf tion of the tuning core. It is apparent of course, that this constant L/R as a function of the frequency is identical to a Q variation which is directly proportional to the frequencysince WL/R=Q, where W is the frequency. Ther attainment of a uniform gain in this'manner, 'hovv ever, limits the overalll selectivity of the circuit by virtue of the fact that the lowest L/R, ratio occurs when the inductance of thetuning co'ilisl at its lowest value, namely, when the tuning core is completely outside theeld of the coil. Sys'- tenis, therefore, using a constant -L/R to effect a uniform radio frequency gain are limited to this low L/R, factor throughout the 'tuning range of the core. Thus a Well-built andfdesigned coil for permeability tuned circuits has a Q, whens the magnetic coreris entirely removed froiiiits field, of approximately 70, while ritsv Q-atonethird thefrequency, which represents an average tuning range for one band of a radio receiver,-

is one-third of '70 or about 23. Since a coil-Q of 23 representsjiJ serious disadvantage fromthe standpointl of selectivity, maintaining the radio frequency response uniform by a constant L/R ratio has not beenentirely satisfactory. vAfurther difficulty in the permeability tuned circuits of this type is found in the lack of adequate image attenuation; high image attenuation being obtained only by limitations `of the circuit to rather selected operating conditions.

It is an'objec'tl of this invention, therefore, to

provide an improved radio frequencycircuitof A further ,objectof this invention istoprovide a permeability tuned radio frequency Aampliercircuit which ischaraoterized bylaY substantially uniform gain and la high Iattenuation .ofthe image frequency over ayvide frequency range.

Anotherr object of this" invention Iis to provide aA permeability tuned radio frequency ramplifier circuit'which'can be operated with equal eiliciency .to provide-a substantially uniform .radio frequency gain .over `a' complete tuningirange regardlessk of whether fthe antenna circuit .gain

1 or the gain at some .other point of .the receiver,

increases 0r decreases with' an increasek in fre,-

quency.

A" further object Aof this invention is to .provide alpermeability tuned circuit .in whicntlie plate circuit impedance yis practically independentof the frequency and in which a high .selectivity is effected over a complete tuning'range.

A feature of .this invention isfound .inthe provision of a permeability tunedtradio frequency amplier vcircuitV including parallel tuned arid seriesl resonant circuits, 'inl whichv a ,pairof tun# ing coils are connected inlseries aidingland.mutu' ally .coupled to one another `in a`r manner-.such that av proper image tracking .isobtained :by a variationv essentially only inthe mutual couplingl reaction-between the'coils.

Yet another .feature ofA this' inveitionosv found inthe provisionV of a permeability Vtuned `radio frequencylamplier circuit in' whichktwoserially connectedcoils arelconcentrically arranged,V but spaced from' each-other so thatvth'eir inductance is varied uniformly and' concurrently; `but .at different rates by the passage; therethrough of'` a magnetic core. The resultant variationfinl -the outer coil is utilized to obtain `a substantially uniform gain with high overall selectivity, over a,VV Wide range. of` frequencies.

Further objects, features and advantages of this invention vwill become-apparent fromv the following description when t'akeninconne'ction with the accompanying drawings, in which:

Fig. l Vis a schematiclcircuit diagramillustrate ing oneembodiment ofithe invention;

Fig'.v 2 is illustratedpsiniilarly .to Fig. -1 and shows a modified form of the' invention;

Fig.; 3 t alsoi illustrated similarly to .Fig- 1-, shows yet another formrof the. invention; i

`Fig. 4 is a sectionalview ofran inductor-suitable for usc, in the4 circuits ofgfFigs. f 1,2l and i 3;

Fie. .5@is a graph .showinefbyf Comparisonscme ofthe operating characteristics-of the circuits of connected either to the total inductance of both i I and 2.

coils in series or only across the outer coil. In

either case, a uniform gain over an entire 'tuning range is simply accomplished by merely selecting a pair of coils of predetermined diameters to provide for a predetermined inductance'variationof' the outer coil. It is contemplated that the inner coil be of a diameter substantially equal to thatr of the tuning core for the coils to obtain as great y a change as possible in its inductance on the adjustment of the tuning core therein. A proper selection of the relative coil sizes is, therefore, restricted to merely a proper selection-of the outer coil. By virtue of this difference in the sizes of the inner and outer coils, their mutual coupling and inductance are varied-on passage of the tuning core therein, to vary the response gain of the circuit in accordance with the predetermined operating curve. Since the insertion of the magnetic core within the field of the coils appreciably increases the L/R ratio of the coil structure, high selectivity over the entire tuning range is provided concurrently with a uniform radio frequency response. This uniform gain can be obtained even though the Q variation of the coils is practicallyropposite to that required for a uniform L/R.r

With the inductance of the inner'coil higher than that of the outer coil,.and with the grid of the succeeding tube connected across the outerV coil, aresonant series circuit is formed across the outer coil. The resonant frequency ofthis series circuit is dependent upon the inductance of the inner coil, the mutual coupling between theA two coils, and the capacitor across the total circuit. Since the mutual coupling coefficient between the coils is a function of the core position and the relativey diameters of the coils, correct image tracking over the entire tuning range of the magnetic core is obtained by a properselection of the coil diameters, this image attenuation being obtained without appreciably affecting the uniform signal gain of the system.

With reference to Fig. 1 of the drawings, the invention is seen toA include a resonant circuit having a variable inductor formed With two coils I and 2 which are serially aiding connected. Coils I and 2 are concentrically arranged with coil 2 positioned within coil I (Fig. 4) but sepaced therefrom as by spacers `I. An adjustable magnetic core 3 is axially movable within coils I and 2, the resonant circuit being completed by an adjustable capacitor 4 which is connected across Y the coils I and 2. A vacuum tube 5, which is preferably a high plate resistance radio frequency pentode is connected to the junction of coils I and 2, with the control grid of a tube 6 being connected across the total inductance of coils I and 2. The vacuum tube 6 may be either a, radio frequency amplifier similar to tube 5, or it may be a mixer tube such as is used in superheterodyne receivers. The signal response from the usual antenna (not shown) is impressed upon vthe control grid of vacuum tube 5. Thus in operation the resonant circuit is tuned to the frequency'of the desired signal impressed on the control grid of the tube 5 by means of the movable magnetic core 3, the voltage developed across the coils I and 2 being applied to the control grid of the vacuum tube 6. By means of the adjustable capacitor 4 the resonant circuit is aligned with the other variable tuned circuits of the receiver, the adjustment preferably' being made at the higher frequencies of the desired tuning range of the receiver, that is, when the 'magnetic core 3 Vis completely removed from within the coils I lWithreference to Fig. 4, it is seen that coil 2 is of substantially the same diameter as the magneticl core 3. Thus as the core 3 is brought into the eldv of. the coils I and 2, the inductance of the coil 2 is changed by a greater proportion .than that of coil I because of its closer proximity to the core 3. With the proper choice or selection of the diameters of the coils I and 2, the change in the circuit impedance in the plate circuit of the'vacuum tube 5 can be varied in such a manner as toiproduce a substantially constant voltage across the grid ofthe tube 5, as will now be explained.

VAs shown int Fig. 1, the voltage applied to the tube 6 may be expressed as where'Eg is the impressed signal on the control grid of tube 5; Gin is themutual conductance of the'tube 5; W isv the resonant frequency of the resonantcircuit; Q is equal to WLt/R in which Lt is the total circuit inductance of coils I and 2, and R is the .effective resistance in series with the inductance Lt; K is the coupling coeflicient between coils I and 2,.and N is the ratio of Lz/Li between the coils in which L1 and Lz represent the inductanceof coils I and 2, respectively. In other words, thevoltage across the grid of the tube 6varies directly vwith the effective plate current change of tube 5. the reactance of coil I and the con factor Q 1+K\/ Since the tube 5 is of the pentode type, the plate current occurring therein for practical purposes, may be considered as independent of the plate load.k In instances where this independence does not exist the coils I and 2 may be constructed tocompensate for ,any variations in the plate current.Y The electrical characteristics of the coils I and 2, namely, the coupling coefcient (K) between the coils, the ratio (N), and thev dissipation factor Q all generally increase, with a decrease in the resonant frequency. This increase is substantially uniform andoccurs as the frequency is varied by adjustingthe position of the magnetic core 3 in the coils I and 2.' With the Q, K and N of the coils I and 2 all tending to increase with a decrease in the resonant frequency, the voltage gain applied on the grid of tube 6 correspondingly increases with such decrease, in frequency. Since theplate load of Vtube 5 is variable directly with the impressedy signal, and mutually related with the coil factor Q(1-}K\/N), the invention contemplates the variation in the reactance of coil I to provide for a substantially uniform voltage gain being applied on the grid control of tube 6. In other words, the reactance WL1 of coil I is varied to decrease uniformly at a rate com'- mensurate with the 'uniform increase in the factor Q(1IK\/N). This variation in the reactance of coil I is accomplished by a proper selection of the diameter and distribution of turns of coil Irelative-tothediameter of coil 2.

S. S. E. .Litz'wire `a diameter of .221' inch and a. length of 1.375

in'ches and is formed .with 247 turns 'effi-44 .Coil I in this instance is .562 inch in diameter and 1.575 inches vlong and vis .comprised vof .47 turns of #38 S. S. Efrwire, 8

turns of which are Wound at 20 turns per inch; 8 turns evenly spaced over .925 inch and 31 turns close..w'ound over a quarter of an inch. VThe tuning core for these coils is .198 inch in .diameter .andv 1.375 inches long; lInfthe practice of this invention, therefore, it is seen that the diameter` of coil 2 is retained substantially equal to the diameter of the core 3. Coil 2, therefore,

has la higher rate ofinductance change than coil I with the reactance change in coil I lbeing dependent entirely upon the variation in the diameter of coil I alone. Y Y

As previously noted, with reference to Fig. 4,

coils I and 2 are concentrically arranged with the coil 2 positioned within Ythe coil I. The magnitude of the inductance in each of the coils! and 2 thus varies concurrently and increasesuni'- formly as the core is moved into the windings. t

increasein the coils I and 2. The voltage applied on the lgrid of tube 6 would thus also be increasing and would rise uniformly as the core 3 proceeded into the coils' I and `2. This conditionfis illustrated by thecurve A in Fig. 5.

Withl the coil I wound on a diameter somewhat larger than that of coil 2, the inductance of coil I as the core 3 is passed into the coils I and 2 increases at a rate less than that of the coil 2. As exemplary of this condition, let it be 'supposed that in movingY the core 3 from a position outside the coils' to va' position fully within th'e coils, that the resonant frequency ofthe tunedcircuit is changed from a certain high frequency lto a ffrequency of only 1/3 such high frequency. The total series inductance of the coils I and 2 is thus increased 9 times. Let itv be further supposed that the extreme positions of the core 3 effects anincrease in the inductance value of coil I vwhich is only three times greater than its minimum value, which minimum value occurs when the core 3 is entirely removed from the coils I and 2. Since ythe vreactance of coil IA varies directly -with the resonant frequency-and the inductance of coil I, it is apparent that the reactance of the coil hasremainedunchanged, because the resonant frequency hasdiminished to 1/3 of its 'original value. This arrangement of the coils I. and 2 thus also provides va rising gain characteristic, as shown by curve B in Fig. 5, .the `reactance of coil I remaining constant and the .rise in gain being effected by the increase in the factor Q(1|K\/N) It is'to be noted, however, thatY the slope of the gain is less than that of the previously cited case where coil- I is wounddirectly about coil 2, as is clearly indicated by a comparison of the curves AandB in Fig. -5.

NowA let it'be assumed that the coil I isof such a large'di'ameter 'that it is notV inuenced by thev position of .the core 3.and is entirely removed from-theeldof the coil2. Y.Under this condi;

tion .theinductance of 'coil I is constant 4so thatv vits reactance decreases as' the frequency decreases, the variation in the reactance now being directly variable with thefrequency. As the core 3, therefore, is moved within the coils I and 2, the voltage gain decreases. Theoperating characteristics of the circuit with thisV selection of the coils I and 2 is illustrated by curve D' in Fig. 5. f

f On a comparative study `of curves A, B and D, in Fig. 5, it is obvious thatr toobtaina uniform signal gain the diameter of coil I must bes'elected so that the reactance of the coil I uniformly decreases as vthe-,frequency decreases and at a rate lcorresponding to the'un i fo`rm increase in the value of the factorrQ 1+K\/N) forthe coils I and 2.V It is Ito 'be understood, of course, that the variation in the reactanceof coil I also varies the factor Q(1-l-K\/N) to a corresponding extent since the voltage applied across the tube 6 is the combined Voltage of both of the coils I and 2. In other words, the variation in the reactance `of coil I to compensate for the increase in value of the factor Q(1+K\/ N) includes also a, consideration of the slight decrease in such, value as effected by the decrease in the reactance of coil I. Since the electrical characteristics of the coils I and 2 are of a known value, it is apparent that this value may be set upY in equational form relative to the reactance of coil I so that the coil performance fora particular operating condition may be predicted. On a proper selection, therefore, of coils I and 2, a substantially' uniform signal gain for the circuit Yis obtained as shown by curve C in Fig. 5.

The circuit diagram shown in Fig. 2 is substantially similar to that of Fig. 1 except that the control grid of tube 5a is connected to the junction of coils `Ia and 2a. The voltage applied on the tube 6a is thus the voltage across coil Ia. rather than the collective or total voltage ofA both coils. In analyzing this circuit the voltage across the tube 6a is seen to be:

wherein all of the factors are similar to `thoser previously Aexplained in connection with Fig'. 1.. When operating the circuit of Fig. 2, using the high frequency end of the tuning spectrum, but

of a greater slope than the curve C in Fig.'5. This is due to the fact that the tying of the grid'of tube 6a to the junction of coils Ia and 2c remo-ves the voltage step-up which normally occurs therebetween. A general loss, therefore,

is effected in theo-verall gain of the circuit of rterxn Q(l-{K\/N) of Equation 1 as the signal frequency decrease-s. l Thus to obtain a uniform gain for the circuit of Fig. 2 the coil Ia Vis reduced in vdiameter so that its reactance or the factor WLla diminishes less rapidly as the frequency decreases. However, to provide a circuit gain in Fig. 2 which is on 'the order of the gain in Fig. 1, both coils lav and. 2a are ,constructed with relatively` larger inductance ;values` than ,the corresponding coilsV l and V2 whichpproducedrthe operating curve C ofv Fig. 5. vThis increased :inductance provides for a substantially high initial voltage developed` by theY coils la A*and Ia'when the core 3a is in its retracted position, at which position the compensating reactance of the coil la will have its greatest value.A With the characteristics of coils la and *2cl of a known value, their performance maybe predetermined vsothat the proper coils for effecting a uniformgain at the tube 6a are readily'selected. Thus in one commercial embodiment of the systemy ofFig. 2 coil la is 1.062 inches in length with a diameter of a half an inchk and comprised of 68 turns of #38 S. S. E. wire wound uniformly at 64.turns per inch. Coil 2a is 1.375 inches long with a diameter of .221 inch and formed With' a progressive universal winding of uniform pitch having 325 turns of 7-44 S. S. E. Litz wire. The tuning core for Vthese coils has a diameter` of .198 inch and a length of 1.375 inches. The additional initial inductance constructed inthe coils la and 2a does not, however, impair the uniformity of the signal response, which is graphically illustrated in Fig. 6 by the curve-B.

With coil la of a diameter such that it is not influenced by the position of the core 3a, and entirely removed from the field of coil 2a, there results the circuit yoperating characteristics shown by the curve' C'v in Fig. 6 which is similar to the'curve D in Fig. 5,the coilsoperating yto effect a voltage gain which decreases as the fre'- quency decreases.

As shown in Fig- 2, the circuit from tube a through coil 2a and condenser 4a forms a series resonance circuit across the coil la, the parallel circuit being comprised of the coils la andv 2a and tuning core 3a and the condenser 4a. The resonant frequency of this series circuit, is higher than the desired signal frequency, and is determined by coil 2a, the adjustment of the condenser 4a and the polarity and magnitude of the coupling coefcient (K) between the coils la and 2a. By a proper selection of the relative diameters of coils la and 2a, a value of K is obtained which combined with the resonance of coil 2a and capacitance 4a acts to short-circuit the plate coil la at image frequencies whereby torsubstantially reduce the image voltage applied to the tube 6a. It is to be understood, of course, that the coupling coefficient (K) varies with the position of the magnetic core 3a within the coils l a and 2a, its value being a minimum with the core entirely removed from the windings la and 2a and a maximum when the core is fully within the windings.4 The selected coils la and 2a, therefore, operate to vary the value of K such that the relation between the resonant frequencies of the series and parallel circuits is substantially correct over the entire frequency spectrum. f

A more clear understanding of the effects of the polarity and magnitude of the mutual inductance factor (K), between the coils la and 2a in providing for high image attenuation is obtainable from the equation where fi is the frequency of image attenuation; fs the frequency of the desired signal, and Lm the total inductance of the coils la and 2a, the factor (K) being of positivepolarity. Let it be assumed .that..the; parallel tuned circuit is tunable over'abroadcast spectrum which extends from 550 kc. to 1500 kc. and that the intermediate. frequency is about 450 kc; Since the local oscillator operates ata higher frequency than the signal frequency the range ofv image response will extendfrom 1450 kc. to 2400 kc., the image frequency being higher in frequency than the signal frequency, by twice the intermediate frequency of the receiven, The ratio ofthe image frequency to the signal frequency at 550 kc. is thus 2.64 and kat 1500 kc. the ratio Yis 1.6. It is seen, therefore, that proper image tracking occurs when the frequency of the series resonant circuit, for the above mentioned frequency range, increases at a rlower rate than the corresponding frequencyof the parallel tuned resonant circuit.

As is apparent from the abovenoted ratio figures the radio of the image frequency-to the signal frequency, or frequency ratio, increases as the signal frequency decreases. From Equation 3 it is seen that this-ratio ,fi/fs 4is directly proportional to Y v t., V2 (Lidl-Kd) Y In one commercial embodiment of the invention, the ratio of f i La was found to be 1.85 with the core A3a retracted and 1.6 with'the core within the coils. This inductance ratio thus decreases with a `decrease in the frequency, which decrease is` opposite in direction to that of the ratioof the image frequencyvto the Signal frequency. However, since the inductance ratio varies but little over the entire frequency spectrum it may be considered as being substantially ,'constant.- Consequently Vonly Vthe Value of the mutualcoupling factor (l-K2) remains to supply the necessary variation for proper image tracking. As was ex#- plained in connection with Equations 1 and 2 relative to uniform gain the coupling coecient K increases with increasing inductance or as the frequency 'decreases'. ThisA increase is in the same direction as that of the ratio' of image frequency to signal frequency. Since the inductance ratio j La and `mutual coupling factor (1'-K2) alone act to 'affect the frequency ratio, this latter ratio is changed only by a change in the coupling factor by virtue of the inductance ratio being substantially xed. Proper image tracking is thus obtained by a selection of the diameters of Acoils la and'2a such that the value of (K) varies with a decrease in frequency to effect attenuation of the image voltage developedY by coil la at a rate which is in accordance with the rate in the change of the frequency ratio.

Since Vthe coupling coefficient (K) increases as the core3a is extended' further into the eld of the coils la and 2a, its slope value is greatest when, the ratio of the diameters of coils la and 2a is a maximum. With the rcoil la wound tightly about the coil 2a, the coupling coefficient therebetween changes butjlittle with the position of the core 3a. Thus high image attenuation by utilization of the coupling coefficient between the coils laV and 2a is obtained only when the coils are relatively spacedfrom each other, so Vthat for .anyV given increment e of` change in the position of. the. cere, the relative resulting chan-.ge in-.the coitflzazwillybedifferenti from the relative change which occurs the coil 2a. Itis ta be noted als@ that the selection of the coils laand 2a may be such that a reasonably satis-'- factory uni-form gain maybeproducedconcurrently with high image attenuation. This operation of the circuit rin Fig. 2 is provided by a consideration of themutual effects of the factors governing the variations both in uniform gain and, image attenuation..y i

In some. cases, however, it may be desirable to have a radiofrequency gain response other than uniform to accommodate the character ofthe antenna gain or othergainA characteristic of a particular receiver. vTormaintain uniform the overall sensitivityV of the receiver vthe radio fre-,- fluencyA amplifier slope, in such instances, would be opposite-A in direction rtc the slope of the antenna gain. As exemplified by` curvesl A,:B. and D in Fig.5, and curves. B' and C. in Fig. 6 for the circuitsof Figs. 1 and 2, respectively, it is, seen that both of the circuits are sufficiently flexible in operation tocompensate for a nonuniform gain in other associated parts of the receiver, such as` the antenna, regardless of Whether this nonfuniform gain may slope up or down asthe frequency increases. However, as

above fully explained, curve A. of Fig. and curve.

C of Fig.v 6. graphically show an extreme operating condition for lthe circuits of Figs. 1 and 2,v respectively. Itis contemplated, therefore, kto use the circuit of Fig. 2 when the receiver sensitivity slopes downwardly, or falls off, as the frequency increases, and the circuit of Fig. 1 when the receiver sensitivityincreases with an in crease in the frequency. It is to be understood, however, that this preference is entirely a matter of reducing the physical size of the coil assembly illustrated in Fig. 4, since the circuits of Figs. 1 and 2 operate with equal eiciency when arranged to compensate for a particular gain characteristic.

The circuit in the modified form of the invention shown in Fig. 3 is substantially identical to that of Fig. 1, except that the connection points for the plate of tube 5 and grid of tube 6 are nterchanged. That is, the plate is connected to coil 2, and the condenser 4 and the grid are connected to the coil I. For the same size coils l and 2 as are used in Fig. 1, the operation of the circuit of Fig. 3 is identical in all respects to that of Fig. 1, so that the response curves in Fig. 5 also graphically illustrate the operating characteristics for Fig. 3. However, this similarity in operation occurs only when the grid input or loading effects of the tube` 6 on the circuit are negligible. At the higher radio frequencies this loading effect may become somewhat appreciable, and the circuit cf Fig. 3, by virtue of the tube E being connected directly to the coil l acts to reduce these effects so that substantially the entire response across. coil I is applied on the tubeA B. Y

The invention thus provides a permeability tuned radio'frequency amplifier circuit which is flexible in operation and readily adapted for application to a plurality 'of receiving conditions. The tuning coils ofthe tuning inductcr are so arranged that the coupling effect therebetween is utilized tol provide a high attenuation of the image frequency and a uniform gain at signal frequencies, while maintaining a high selectivity.

It is to be understood that only preferred embodiments of the present invention have been illustrated and. described herein and that .modifications and alterations may be made in these embodiments, which are within the full intended scope .ofthe invention asy defined `by the appended claims. Y

I claim:v

1. In atuning systemY having a capacitive element and a vacuum tube witha control grid; the combi-nation of a pair of inductive, elements sub stantially equal in length connected with said y Acapacitive element to form a parallel resonant circuit, said grid being connected in said circuit 2 5 to simultaneouslyy and uniformly vary the mutual inductanzce'of said coils and' the 'reactance of said other coiltol provide` for the application of a substantially uniform voltage Von the grid of said vacllllmtube. l' r '12. In a tuningsystem having a capacitive element anda vacuum tube with a control grid, the

f combination of a pair of inductive` elements of nected and Aconcentrically arranged with one insubstantially the saline, length serially aiding conductive element inserted entirely within the other but spaced therefrom radially, said inductive elements having a mutual inductance and being connected with said capacitive element to form a parallel resonant circuit, with the voltage developed across said other inductive element being applied on said control grid, and a series resonant circuit including said capacitive element and said one inductive element, and means for tuning said two resonant circuits including a tuning core axially movable within said one in-t ductive element and uniformly varying the mutual inductance of said two inductive elements to provide for the application of a uniform response gain on said control grid.

3. In a tuning system including a capacitive element and a pair of vacuum tubes each of which has a control grid and a plate, the combination of a pair of mutually coupled inductive elements of substantially equal length, means for mounting said elements so that they are concentrically arranged in a spaced relation with one. inductive element entirely within the other lenghwise thereof and having a junction there-V between, a parallel resonant circuit including said capacitive element and said pair of inductive elements, a series resonant circuit including said one inductive element andY capacitive element, with said two resonant circuits being resonant at different frequencies, the control grid of one ofsaid tubes and the plate of the other tube being connected to said coil junction, with the voltage applied on the grid of said one tube being the voltage developed by said other inductive element, and means for tuning said two circuits including a tuning element movable within said inductive elements along their common axis and concurrently varying the inductance of each of said inductive elements and the reactance of said Y other inductive element at a uniform but different rate toeifect a proper tracking between the wisethereof, Va parallel resonant circuit includingk said capacitive element and said pair of inductive elements, a series resonant circuit including said capacitive element, said one inductive=element and a junction therebetween, with the inductance of said other inductive element beingapplied on the grid of one of saidzvacuum tubes, and the plate ofrfsaidV other vacuum tube being connected to said junction, andmeans for tuning said two resonant circuits including a tuning core axially movable within saidKV one inductive element to uniformly and simultaneouslyvary' the inductance'of said two coils overtheir length to provide for a substantially'uniform response gain being appliedon the grid of said one' tube.

` 5. A tuning system for a'radio receiver having a pair of inductive elements serially' aiding connected and lconcentrically arranged ina spaced relation with one thereof within the other, a parallel resonant circuit including said pair oi inductive elements and a capacitive element connected in Aseries therebetween, a series resonant circuit including said capacitive element and said one inductive element, and means for tuning saidtwo resonant circuits including a magnetic core movable within said one inductive element and axially relative to the other inductive element to simultaneously vary the reactance of said other inductive element and the inductance of said two inductive elements over their complete lengths, withthe reactance of said other inductive element and the 'mutu'al'inductance between said two inductive elements providing for a substan-k tially uniform response gain with high selectivity.

'6. In a tuning system having a tuning circuit including a capacitive element and a vacuum tube with a control grid, the combination of a pair of 'inductive elements of substantially the same length serially aiding connected and concentrically arrangedwith onev inductive element inserted entirely within theotherv but spaced thereiromradially, said'inductive elements having a mutual inductance, and said one inductive element and said capacitive element being connected together in series across said other inductive element, with said control grid being connected in said circuit so as to have at least the voltage developed by said other coil applied thereon, and means for tuning said lcircuit including a tuning core movable within said one inductive element along the axis common to both of said inductive elements tofsimultaneouslyA and uniformly vary the Vmutualginductance of said coils and the reactance of said other coil to providefor the application of a substantially uni'- form voltage gain on the control grid of said vacuum tube, 9

RINALDO DE COLA. 

